Ford vs GM towing capacities - Older model trucks...

Love my Ford..Mine is a near mint 100% garaged, '99 F150, RWD, Lariat extended cab, 4.6 with the towing package and 3:55 rear I ordered new from the factory when I had money...You know - Before I got married.
I get a lot of compliments and looks at it...(The truck and the wife, but the truck uses less money and gives me less headaches.)
Manual say this configuration has towing capacity of 8K. I've been towing a 29' 7K lb max travel trailer for years with the family. I keep it locked out of OD with the button on the gear shift. Sucks down gas, but tows the load well. Last year I towed it 13 hours straight to VA, and over the hills around Allentown PA on Rt 78 (for those that know the area) maintaining 60-65, without ever dropping below 55 on the inclines and only occasionally dropping back a gear. All and all, good towing. I only use the truck when I need a truck - not a daily driver, so it mostly tows this rig around, gets the Christmas trees, Home Depot runs, etc...that's it. Maybe 2-3K miles a year max.
Here's the thing....I am going upstate NY this year, but the kids are getting bigger and it's getting a bit crammed in that extended cab back there, so I thought I'd use the Ford more for our tons of local camping trips, but borrow my Pop's truck for the big one as it has more back seat room. His is a '98 GMC Sierra 2500... RWD, 3/4 ton, good condition, less miles etc. On a whim I checked the towing capacity on it and am still in shock that it has less than mine. The manual says the max tow capacity with his factory installed tow set up GMC 2500, 5.0L with 3.73 is 6,500 lbs.... WTF??!
Was GM being a little cautious in their ratings back then? Was Ford being too excessive? More than a little surprised that a GM 3/4T 2500 would have less LISTED capacity than the 1/2T F150... I know things have changed a lot since then...Hell - I think they rate the new F150 at around 10K or some crazy number now, but sheesh...
After towing with my F150 and having no problems, I would like to think I can tow the same trailer with a GM 2500 for this one trip. Sure, I'd LOVE to get a new F250 SuperDuty, but I can't afford one or justify it. I have my main beater daily car with 180K on it, a new reliable one for the wife and car trips, two kids, mortgage, etc. Can't justify a new truck payment for 2K miles a year....
Ah well - Maybe in a few years I can save enough for a used one that I can pay cash outright for....
Anyone experienced with both Ford and GMC to know what's up with the capacities?
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